I don't have an old computer to make an air gap. I do have a raspberry pi from 2012 though. If I made a new SD card with fresh OS would this be safe enough to make cold wallets on? It would be easy to hide, also.
Sure! Boot the Raspberry, change admin password (keep this safe!) turn off the wifi and bluetooth.
Turn wifi and bluetooth off:
sudo nano /boot/config.txt
Reboot and NEVER turn on the internet again on this Raspberry.
Note all the software you think you will need (Monero, gpg and maybe KeePassX). Download them on your online mac/pc, checksum all the files and signatures, move to usb stick. Plug into Raspberry and checksum again all the files. Install what you need, store your password for the Monero Wallet in KeePassX (maybe also use an extra usb-stick as a KeePassX-keyfile).
Some more information on making a cold wallet and a view-only wallet on your online machine to check the balance: